The formation of parts from consolidated mats, made from co-mingled reinforcement fibers and thermoplastic filaments, includes heating the mats to a melting temperature of the thermoplastic filaments. However, the mats, including the thermoplastic filaments, have a tendency to expand and bulk up during the heating phase prior to the shaping phase. Such expansion and bulking may make closure of a part-forming mold difficult or even impossible. Additionally, expansion and bulking of the mat may result in insufficient pressure to form the part. Compensating for the insufficient pressure, by increasing the pressure applied to the part, may increase the risk of collapsing the core of the part.